[en] European eels (Anguilla Anguilla), an endangered species with high economic value, exhibit very unusual features in their multistage catadromous life cycle at different life stages. The reproduction of eel takes place in the Sargasso Sea. Leptocephalus larvae use ocean currents (mainly the Gulf Stream) to migrate to European coastal areas, and then metamorphose into glass eels probably as they reach the continental shelf. Glass eel then, enter estuaries to join the fresh waters network.
Knowledge about glass eel biology and migration has mainly increased in the eighties, and it is now well established that the activity during the estuary migration depends mainly on tidal cycle, even if lunar and circadian rhythms and/or direct influence of light and tidal coefficient may interact. Different migratory tactics are suspected on the basis of recent laboratory experiments (Bolliet & Labonne, 2008; Bureau du Colombier et al., 2009), notably the existence of individuals swimming only with the current, others swimming with and against the current, and others presenting a weak degree of activity. The propensity to migrate and glass eels energy content would be linked and may result in different migratory behaviours possibly leading to estuarine settlement (Bureau du Colombier et al., 2007; 2009).
Unfortunately, few are known about the glass eel swimming behaviours in estuaries, a crucial point to better understand their estuarine migration: such knowledge would help to characterize the migratory tactics and to better understand the link with energy status.
The videotracking system [EthoVision Color-Pro 3.1 (Noldus Information Technology)] which allows to measure automatically the movements of individuals represents an interesting tool to progress in this questioning. Nowadays, video tracking to measure fish behaviours is relatively frequent, but is rather rare in the ecological studies. Moreover, tracking a transparent animal, under water current conditions is a real challenge, particularly at night.
To detect the individuals, several eels are marked by a fluorescent VIE tag (Visible Implant Elastomer, Northwest Marine Technology Inc.). VIE do no affect growth, survival and behaviour of tagged glass eels. Initially developed to improve the identification of individual tags in mark-recapture studies of aquatic animals, VIE tags are used here for a new extreme application in ethometry: the study of behaviours of transparent animals in weaker luminosity and in the dark by automatic tracking technology.
The individual identification is based on the fluorescent color (red, blue, yellow and green) emitted by the VIE tag injected in the muscle mass on the basis of dorsal fin. The detection of the four marks is excellent. It allows measuring the activity (notably path and speed) of each tagged individuals as a function of tidal and nycthemeral rhythms in the same chronotron (circular aquarium simulating the estuarine conditions) and spatial use at a wide time scale from second to week. Another advantage is the possibility of tracked animals to go out the analysed arena without error of identification when it appears again in the analysed zone.
This multitracking method by fluorescent VIE tags could be applied to a large number of species (crustaceans, fish, holothurians, amphibians, …).

Acknowledgements: This work was financially supported by FNRS-FRS (National fund of Research, Belgium) (project n°2.4617.08, n°2.4569.06, n°2.4569.10F), by Hubert Curien Tournesol project and by an EGIDE Grant. We thank J-C. Aymes for its help in testing the video tracking software and system. We are also highly grateful to Jacques Rives for its help in settling and monitoring the experiments.